The present invention relates to a pneumatic tire, and more particularly to a heavy duty radial tire having an improved belt structure by which the tire strength (breaking energy) is effectively increased while achieving a belt weight reduction.
In a belted radial tire for heavy duty use such as truck/bus tires, it is very important to improve the hoop effect of the belt to withstand a high inner pressure and a heavy tire load. Therefore, such a belt layer generally comprises at least three plies of steel cords.
In such heavy duty radial tires, it is also necessary to increase the breaking energy (plunger energy) for the belt in order to prevent the burst of the tire. A burst of a tire is caused when its belt plies are cut by a sharp object on the road surface, such as stones and rocks. Indeed, Japanese Industrial Standard (JIS-D4230) in other countries requires the breaking energy to score above a regulated value in the tire breaking test.
Hitherto, therefore, the breaking energy is increased by using thicker steel cords for the belt and/or increasing the cord count for each of the belt plies.
However, if the cord diameter and cord count are simply increased, the tire weight and manufacturing cost are greatly increased, and the dynamic tire performance is reduced. Further, the breaking energy sometimes unexpectedly decreases.
Therefore, the present inventor has made various studies and tests, with the following discoveries.
(1) The cords of the second and third plies mainly work as a hoop, and the cords of the first ply works to control a cord movement of the second and third plies. PA0 (2) When a tire treads upon a sharp object, the belt plies deflect towards the radially inside of the tire and at the same time the angle of the belt cords are changed. If the first ply strength is excessively high, the cord movements of the second and third plies are excessively controlled and their cord angles cannot be changed. As a result, the second and third ply cords are liable to be broken. PA0 (3) When the belt plies deflect toward the radially inside the ply locates farther from the sharp object and a larger tensile deformation occurs. Accordingly, the second ply is broken easier than the third ply. On the other hand, the first ply is difficult to break when the cord angle is large and therefore it is soft against bending.
It is therefore, an object of the present invention to provide a heavy duty radial tire, in which the breaking energy is increased without increasing the tire weight.
According to one aspect of the present invention, a heavy duty radial tire comprises a carcass extending between a pair of axially spaced bead portions of the tire, and a belt disposed radially outward of the carcass in a tread portion of the tire, the belt comprising at least three plies including first, second and third plies disposed in this order from the carcass to the radially outside thereof, each belt ply being made of steel cords laid parallel with each other. With respect to the tire equator, the inclining direction of the cords of the first belt ply is the same as the inclining direction of the cords of the second belt ply, but is reverse to the inclining direction of the cords of the third belt ply. The angle of the cords of the first belt ply to the tire equator is 35 to 80 degrees, the angle of the cords of the second belt ply to the tire equator is 15 to 30 degrees, and the angle of the cords of the third belt ply to the tire equator is 15 to 30 degrees. The total of the ply strength of the second belt ply and the ply strength of the third belt ply is 3.4 to 10.0 times the ply strength of the first belt ply, wherein the ply strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply.
According to another aspect of the present invention, a heavy duty radial tire comprises a carcass extending between a pair of axially spaced bead portions of the tire, and a belt is disposed radially outward of the carcass in a tread portion of the tire, the belt comprising at least three plies including first, second and third plies disposed in this order from the carcass to the radially outside thereof, each belt ply being made of steel cords laid parallel with each other. With respect to the tire equator, the inclining direction of the cords of the first belt ply is the same as the inclining direction of the cords of the second belt ply, but reverse to the inclining direction of the cords of the third belt ply. The angle of the cords of the first belt ply to the tire equator is 35 to 80 degrees; the angle of the cords of the second belt ply to the tire equator is 15 to 30 degrees; and the angle of the cords of the third belt ply to the tire equator is 15 to 30 degrees. The ply strength of the second belt ply is 1.05 to 2.0 times the ply strength of the third belt ply, wherein the ply strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply.
In a first aspect, therefore, the movements of the cords of the second and third belt plies are appropriately controlled, and the cords are prevented from being cut, and the breaking energy for the belt can be increased.
Similarly, in a second aspect, as the second belt ply is increased in its strength S2, it becomes possible to withstand its large tensile stress, and the breaking energy can be increased.
Further, when those two aspects are combined, the breaking energy is remarkably increased.
In either case, as the cord angles of the second and third belt plies is relatively small, those plies display a tight hoop effect, and the cords of the first belt ply appropriately control the movements of the second and third belt ply cords. Further, the cords of the first-third plies form a truss structure to provide a desirable rigidity for the belt.